xen/events: Add the hypervisor interface for the FIFO-based event channels

Add the hypercall sub-ops and the structures for the shared data used
in the FIFO-based event channel ABI.

The design document for this new ABI is available here:

    http://xenbits.xen.org/people/dvrabel/event-channels-H.pdf

In summary, events are reported using a per-domain shared event array
of event words.  Each event word has PENDING, LINKED and MASKED bits
and a LINK field for pointing to the next event in the event queue.

There are 16 event queues (with different priorities) per-VCPU.

Key advantages of this new ABI include:

- Support for over 100,000 events (2^17).
- 16 different event priorities.
- Improved fairness in event latency through the use of FIFOs.

The ABI is available in Xen 4.4 and later.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

include/xen/interface/xen.h

@@ -281,12 +281,6 @@ struct multicall_entry {
 };
 DEFINE_GUEST_HANDLE_STRUCT(multicall_entry);
 
-/*
- * Event channel endpoints per domain:
- *  1024 if a long is 32 bits; 4096 if a long is 64 bits.
- */
-#define NR_EVENT_CHANNELS (sizeof(xen_ulong_t) * sizeof(xen_ulong_t) * 64)
-
 struct vcpu_time_info {
 	/*
 	 * Updates to the following values are preceded and followed